This invention relates to a solid oxide fuel cell generator and, more particularly, to a high temperature generator having spaced apart rows of spaced apart, annular, axially elongated fuel cells electrically connected in a series-parallel array by electrically conducting members.
High temperature solid oxide fuel cell generators are employed to convert chemical energy in a fuel gas, such as natural gas or a combusted gas containing hydrogen, carbon monoxide, methane and the like, into electrical energy. Thus, for example, gases from a fossil fuel power plant at temperatures over about 700.degree. C. may be oxided by air in such a generator to produce electrical power and also environmentally acceptable water vapor and carbon dioxide. However each individual fuel cell in a generator only produces an open circuit voltage of about one volt. Also, each cell is subject to electrode activating energy losses, electrical resistance losses and ion mobility resistance losses which reduces its output to even lower voltages. Accordingly, a generator has many fuel cells electrically connected in series to produce the desired voltage. In addition, many fuel cells are electrically connected in parallel to produce the desired current. A generator designed to produce 500 KW of power, e.g., would literally have thousands of such fuel cells.
U.S. Pat. Nos. 4,431,715; 4,490,444; 4,520,082; 4,699,852; 4,791,035 and 4,894,297 generally disclose the structure and operation of generators employing spaced apart rows of spaced apart, annular, axially elongated fuel cells wherein the fuel cells of one row are electrically connected in series with the fuel cells of adjacent rows by axially extending felt strips. In addition, the fuel cells of each row are electrically connected in parallel by axially extending side felts. These patents are incorporated by reference for their disclosures. In the generators, each fuel cell is supported on a porous support tube which may be, e.g., a hollow calcia-stabilized zirconia tube having a wall thickness of 1 mm. An electrode (which will be assumed to be an air electrode for purposes of illustration) is deposited on the periphery of the support tube. A known air electrode comprises a 50 to 500 .mu.m thick composite of doped or undoped oxides or a mixture of oxides of the perovskite family, indium oxide, rare earth oxides and oxides of cobalt, nickel, copper, iron, chromium and manganese. A nonporous solid electrolyte is deposited on the periphery of the air electrode. A known solid electrolyte comprises a 20 to 50 .mu.m thick yttria-stabilized zirconia structure which substantially encompasses the air electrode. A small portion of about one radian or 50 degrees of the peripheral area of the air electrode extends axially along its length in contact with an interconnect material which may be an oxide doped (e.g., calcium, strontium or magnesium) lanthanum chromite film. The interconnect material extends outwardly through the solid electrolyte and a fuel electrode which is deposited on the periphery of the solid electrolyte. A known fuel electrode comprises a 50 .mu.m thick cermet such as nickel zirconia. The fuel cells may be electrically interconnected by highly porous felt strips or other electrically conducting members extending between the outer fuel electrodes of adjacent fuel cells to establish parallel electrical connections or extending between the outer fuel electrodes of one row of fuel cells and the inner air electrodes (via the interconnect materials) of fuel cells in an adjacent row to establish series electrical connections. The felts may be sinter bonded nickel fibers where the fuel gas flows over the fuel cells in a plenum and the air (or other oxygen-containing gas) stream flows through the support tube. If the flows are reversed such that air flows over the fuel cells, the felt may be made from conductive oxide fibers such as indium oxide and the like.
FIG. 1 shows an array of 6 (series) .times.3 (parallel) connected fuel cells 1-18 in the plenum 20 of a solid oxide fuel cell generator 22. Cells 1-6 are connected in series, as are cells 7-12 and cells 13-18. Axially extending felt strips 24 connect the adjacent fuel cells in series and axially extending side felt strips 26 connect the adjacent fuel cells in parallel. In addition, felt strips 28 extend between fuel cells 1, 7 and 13 and bus 30 and felt strips 32 extend between fuel cells 6, 12 and 18 and bus 34.
FIG. 1 shows that there are at least 33 felt strips 24, 26, 28 and 32 employed in this array. If the fuel cells 1-18 are 50 cm long and the felts 24, 26, 28 and 32 are about 15 cm long (which is a known design), then three times the 33 felt strips 24, 26, 28 and 32 (or ninety-nine) must be employed in the 6.times.3 array. Obviously the assembly of such an array is labor intensive and arduous. Each one of the felts 24, 26, 28 and 32 must be individually fabricated and applied. Additionally, the series felts 24, 28 and 32 and side felts 26 are applied in planes perpendicular to each other. This requires several step-by-step operations in applying the felts 24, 26, 28 and 32, including the application of nickel slurries, the application of the felts 24, 26, 28 and 32 themselves and drying steps. There are about a dozen steps in all.
In use, the performance of an array of fuel cells may change over time because of the changing condition of the strips. As FIG. 1 shows, the strips 24, 26, 28 and 32 cover much of the outer surface of each fuel cell 1-18. The strips 24, 26, 28 and 32 may be in contact with up to 80% or more of the fuel (i.e. outer) electrode. When the fuel gas flows over the fuel cells 1-18, the felt strips 24, 26, 28 and 32 may impede the flow of water vapor and carbon dioxide from the fuel cells 1-18, which tends to reduce the power density of the array. Also, the strips 24, 26, 28 and 32 may trap impurities, which further tends to reduce the flow of water vapor and carbon dioxide. In addition, uneven thermal stresses tend to develop in an array because the heat generated by the fuel cells in the peripheral portions of a plenum 20 of a generator 22 can be dissipated by conduction faster than can the heat generated by the fuel cells in the interior portions of the plenum 20. Further if a felt strip 24, 26 or 28 becomes non-conductive then an entire vector of fuel cells may be rendered substantially inactive.